1. Field of the Invention
The present invention relates to a variable-discharge high pressure pump which varies the discharging quantity of fuel on the basis of the timing of energization to (actuation of) a solenoid valve.
2. Description of the Prior Art
FIG. 5 is a cross-sectional view showing a prior variable-discharge high pressure pump disclosed by, for example, Japanese Unexamined Patent Publication No. 4-36073. The prior variable-discharge high pressure pump 106 is equipped with a cam shaft 111 which rotates with a crank shaft of an engine. Around the cam shaft 111 there is defined a cam surface 112 which rises and falls in accordance with the rotation of the cam shaft 111. The cam surface 112 includes two heights and is of the so-called two-high type. The cam shaft 111 is located within a cam chamber 114 existing at a lower portion (a lower side in FIG. 5 when being mounted on a motor vehicle) of a housing 113. The housing 113 accommodates a substantially cylindrical room 115 where a tappet 116 is supported to be allowed to reciprocate in the directions of the axis of the cylindrical configuration. The tappet 116 is, at its cam surface 112 side, provided with a cam roller 118, and the cam roller 118 rolls in response to the rotation of the cam shaft 111 as it turns being while in contact with the cam surface 112, causing the tappet 116 to shift up and down in accordance with the variation of the cam surface 112.
Furthermore, in an upper portion (an upper side in FIG. 5 when being mounted on a motor vehicle) of the housing 113, a cylinder 120 is fixedly provided in a state that its central portion invades the interior of the room 115. In the central portion of the cylinder 120, there is made a plunger sliding section 120a awhich is a bar-like hole having a circular cross section, and a plunger 127 is inserted thereinto to be allowed to reciprocate in its axial directions. This plunger 127 has a bar-like configuration with a circular cross section and its lower end portion is in connecting relation to the tappet 116, so that in response to the reciprocating action of the tappet 116, the plunger 127 reciprocates in the interior of the plunger sliding section 120a. A pump chamber 121 is defined in a portion surrounded by the top surface of the plunger 127 and an inner circumferential surface of the plunger sliding section 120a of the cylinder 120. The top surface of the plunger 127 compresses the fuel within the pump chamber 121.
In the cylinder 120, a fuel introduction means 128 is placed for the purpose of introducing fuel into the variable-discharge high pressure pump 106. In addition, accommodated therein is a solenoid valve 130 being under the energization control of a control unit (not shown). The solenoid valve 130 is installed in such a manner that a male screw portion 130a made in its own outer circumference is engaged with a female screw portion 120b made in a fitting section of the cylinder 120 and tightened to be fixed in a state that its bottom portion comes into contact with a step portion of the fitting section. This solenoid valve 130 is for opening and closing a fuel supply passage 131 for leading fuel to the pump chamber 121. For example, when the solenoid valve 130 is in the energized or driven condition, the fuel supply passage 131 is in the closed state, whereas the fuel supply passage 131 comes into the open state in response to stopping the energization thereto. Further, in the cylinder 120, a fuel discharging means 132 is installed to discharge the fuel compressed in the pump chamber 121 toward the exterior of the variable-discharge high pressure pump 106. The fuel discharging means 132 has a check valve 133 therein.
Secondly, a description will be made hereinbelow of an operation of the variable-discharge high pressure pump 106. The fuel is supplied through the fuel introduction means 128 and the fuel supply passage 131 into the pump chamber 121. Further, when the solenoid valve 130 is actuated by the control while the plunger 127 rises, the solenoid valve 130 closes the fuel supply passage 131. Thus, the fuel within the pump chamber 121, compressed in accordance with the rise of the plunger 127, stops to escape through the fuel supply passage 131 and reaches a high pressurized state. When the pressure within the pump chamber 121 rises to open the check valve 133, the pressurized fuel is discharged through the fuel discharging means 132 to the external.
In general, in the case of the four-cylinder engine, the variable discharge high pressure pump needs to discharge the fuel four times during one revolution of the engine crank shaft. For this reason, in the case of the use of such a variable-discharge high pressure pump 106 with the two-high cam, one engine requires two variable-discharge high pressure pumps 106. Recently, in order to increase the discharge quantity of the variable-discharge high pressure pump to decrease the number of the pumps, the trend is toward further increasing the number of cam heights on the cam surface 112 to increase the number of sliding movements of the plunger 127 per one revolution of the cam shaft 111.
In the case of the prior variable-discharge high pressure pump 106 thus constructed, the inner circumferential surface of the plunger sliding section 120a of the cylinder 120 is required to have an extremely high processing accuracy because of the sliding action of the plunger 127 and the compression of fuel. In addition, the inner circumferential surface of the plunger sliding section 120a is required to be free from the deformation hindering the sliding movements of the plunger 127 after the completion of the final assembling. More specifically, it is necessary that the bending deformation of the plunger sliding section 120a in the axial directions or the inward projection of the inner surface of the plunger sliding section 120a does not take place. Thus, the cylinder 120 is wholly constructed to have a high rigidity in order to prevent the deformations of the fitting portions due to the attachments of the solenoid valve 130 and the fuel discharging means 132 from having influence on the plunger sliding section 120a.
FIG. 6 shows a structural analysis result of the cylinder 120 after the completion of assembling of the prior variable-discharge high pressure pump 106, where only one side of the cross section of the cylinder 120 is illustrated but the other side is omitted for brevity. This illustration signifies a state of the deformation of the cylinder 120 caused by the force occurring due to the fixing of the solenoid valve 130 onto an upper portion of the cylinder 120. The solenoid valve 130 is mounted in such a manner that the male screw portion 130a made in its own outer circumference is engaged with the female screw portion 120b made in the fitting section, and fixed so that its bottom surface is brought into contact with a step portion of the fitting section. For this reason, there develop the force Q applied to enlarge the fitting section and the force P exerted to press the fitting section upwardly due to the reaction against the contact of the bottom surface therewith. In this instance, the force P is approximately 3600 kgf while the force Q is approximately 1300 kgf. These forces P and Q make the plunger sliding section 120a of the cylinder 120 deform as indicated by a dotted line in the illustration. As described before, the cylinder 120 is made to exhibit a high rigidity so that the deformation due to the installation of the solenoid valve 130 and the fuel discharging means 132 does not have influence on the plunger sliding section 120a. Accordingly, the deformation of the plunger sliding section 120a is small as 0.6 m as illustrated in a state with the fixed solenoid valve 130.
However, in the case of the prior variable-discharge high pressure pump thus constructed, in the cylinder 120 there are formed the installation section for installing the solenoid valve 130 and the fuel discharging means 132 and the fuel supply passage 131 for leading the fuel, which makes its configuration complicated. In addition, the cylinder 120 is designed to have a high rigidity to prevent the force due to the fitting of the solenoid valve 130 and the fuel discharging means 132 from having influence upon the other sections. Further, the plunger sliding section 120a of the cylinder 120 requires a high accuracy because of permitting the reciprocating movement and the compression of fuel. For these reasons, difficulty is experienced to easily manufacture the cylinder 120 with a high accuracy.
On the other hand, in proportion to the recent tendency to increase the number of heights of the cam, the sliding speed of the plunger 127 becomes higher an is, therefore, more prone to the seizure. Accordingly, the processing accuracy of the plunger sliding section 120a needs further improving, besides the plunger sliding section 120a requires a high lubrication performance.